1. Field of the Invention
The present invention relates a printed circuit board (hereinafter referred as PCB) and more particularly, to a flexible printed circuit board (hereinafter referred as FPC), which can prevent breaking of the circuit by changing the trace of the lead wires.
2. Description of the Related Art
FIGS. 1 and 2 show a conventional FPC. According to this design, the FPC comprises a flexible substrate 1, a plurality of lead wires 2 that are arranged on the surface of the flexible substrate 1 and have respectively one end terminating in a contact 2a, a protective layer 3 that is formed on the surface of the substrate 1 and covered over the lead wires 2 and has two openings 3a for exposing respectively a pair of contacts 2a having bipolar polarity, and solder pads 4 respectively formed on the contacts 2a for the bonding of an electronic device 5.
As shown in FIG. 1, the lead wires 2 extend orthogonally across a folding line L1 of the flexible substrate and electrically connected to the electronic device 5 through the solder pads 4. Because the flexible substrate 1 and the solder pads 4 are made of different materials having different hardness, i.e. the hardness of the former is less than that of the latter, when bending the FPC along the folding line L1, a stress may be concentrated on the bonding area between the lead wires 2 and the solder pads 4, thereby causing the solder pads 4 or the lead wires 2 to break.
FIG. 3 shows another structure of FPC according to the prior art. According to this design, the FPC comprises a flexible substrate 6, two lead wires 7, and a light-transmittable protective layer 8. The flexible substrate 6 has a bonding zone 6a and a folding zone 6b. The lead wires 7 each have a plated through hole 7a disposed at one end within the bonding zone 6a. The other end of each lead wire 7 extends to the folding zone 6b. The protective layer 8 is covered on the surface of the flexible substrate 6, having an opening 8a corresponding to the plated through hole 7a of each lead wire 7. After bonding of the bonding zone 6a with another electronic device, for example, a PCB 9a, a part of the cured solder material 9b is formed on the flexible substrate 6 corresponding to the opening 8a. Because it is difficult to accurately perform the soldering operation, the soldered electronic device may be biased from the designed solder position. In order to prevent a bad result due to inaccurate bonding operation, each opening 8a is made relatively greater for tolerance. However, in actual bonding operation, a part of each lead wire 7 may lay open to the outside. When the folding zone 6b of the flexible substrate 6 is turned back along a folding line L2 relative to the bonding zone 6a, a stress may be concentrated on the exposed part of each lead wire 7 to cause the break of the exposed part of each lead wire 7 so as to further cause malfunction of the PBC due to the open circuit of the lead wires 7.
Therefore, it is desirable to provide a FPC that eliminates the aforesaid drawbacks.